This invention relates to new photohardenable elements and to new processes for forming images. More particularly, it relates to photohardenable elements comprising double negative-working photohardenable strata. 2. Description of the Prior Art
Photohardenable elements are well known in the art for use in the graphic arts or printing fields. The photohardenable element is exposed imagewise, e.g., through a separate light stencil or photographic image-bearing transparency, to actinic radiation which causes the hardenable component of the composition to harden, e.g., photopolymerize or photocrosslink, and become substantially insoluble in the exposed areas. Following exposure, the element is treated to remove the soluble, unhardened composition in the unexposed but not the exposed areas. The resulting photohardened relief is useful as a planographic printing plate, a direct resist image, or a lithographic film.
In the imagewise exposure, the photographic transparency is usually placed in contact with the surface of the photohardenable element with this assembly in a vacuum printing frame. High vacuum is applied to insure uniform contact of the transparency with the surface of the photohardenable element, and the assembly exposed to a source of actinic radiation.
Frequently, when using this assembly, difficulties are encountered in maintaining adequate, uniform contact between the transparency and the surface of the photohardenable element. Although, usually, a vacuum of approximately 25 inches (63.5 cm) is used, the temperatures prevailing during exposure tend to expand entrapped air and vapors evolved from the photohardenable element, resulting in poor, nonuniform surface contact; poor contact can lead to distorted photohardened images.
Image-forming elements described in U.S. Pat. Nos. 3,245,793 and 3,730,717 which utilize a silver halide layer as overcoats on a photohardenable layer were devised to overcome the aforementioned problem. While effective for this purpose, the elements have certain disadvantages. They require the use of an expensive precious metal, silver, and are very susceptible to light other than so-called "safe" light. U.S. Pat. No. 3,245,793 requires the full silver halide development procedure, e.g., development and fixing. U.S. Pat. No. 3,730,717 utilizes photodevelopment with, however, special silver halide emulsions and photopolymer initiated systems.